+1 on the centering washers on a sufficient tightness to the frame and using cone wrench or brake centering flat wrench to center. The other is to check cable housing run to make sure you aren't artificially pushing down or pulling up on the brake arch excessively when turning the handlebars. I used to also take small 1cm sections of teflon brake cable liner and grease the spring arms and slide that on where it presses the brake arch to see if this made any difference.